Method of detecting and locating an electrostatic discharge event

ABSTRACT

A method of locating an electrostatic discharge event in a predetermined  ucture by sensing electromagnetically that an electrostatic discharge event has occurred and producing a trigger signal from the electromagnetic sensing to control a recording device for the starting of time, utilizing a plurality of acoustic detectors about the structure for sensing the acoustic of the electrostatic discharge event, determining the time elapsed between the starting of time by the electromagnetic sensing and the signals sensed by the acoustic detectors and utilizing the information gained from the times sensed and the predetermined locations of the sensors to determine the location of the electrostatic discharge event in the predetermined structure.

DEDICATORY CLAUSE

The invention described herein may be manufactured, used, and licensedby or for the Government for governmental purposes without the paymentto us of any royalties thereon.

BACKGROUND OF THE INVENTION

In the past, it has not been possible to accurately locate electrostaticdischarges even though it is important to accurately locate the site ofelectrostatic discharges that occur in systems or materials that aresensitive to electrostatic discharges. Sensitive electrostaticdischarges that need to be identified involve systems that includemissile systems that can be damaged by electrostatic discharges andpropellants that can be inadvertently detonated by an electrostaticdischarge event. These events are electrostatic arcs that are caused bycharge separations. The charges may become separated by friction,propellant separation, or by induction fields in voids. These eventsgenerally occur subsurface and are not detectable optically. The factthat an event has occurred can be detected electrically, but to locateit electrically requires a very dense grid of sensors which is costprohibitive, difficult to use and perturb the charge distribution to bemonitored. Thus, it can be readily appreciated that a need exists for arelatively inexpensive, easy to use, and accurate way to detect andlocate electrostatic discharge events.

Therefore, it is an object of this invention to supply a method by whichan electrostatic discharge event can be detected and located.

Another object of this invention is to utilize an electromagnetic methodfor detecting the event and an acoustic method for producing informationfrom which the site of the electrostatic discharge can be located.

Still another object of this invention is to provide a method thatutilizes means that are readily available, relatively simple, relativelyinexpensive and have the ability to provide sufficient information toaccurately locate an electrostatic discharge event.

Other objects and advantages of this invention will be obvious to thoseskilled in this art.

SUMMARY OF THE INVENTION

In accordance with this invention, a method of detecting and locating anelectrostatic discharge event is provided in which an electromagneticdetector on the structure of the device that has the electrostaticdischarge event occuring there at is utilized for detecting that theevent has occurred to start the counting of time, three acousticdetectors placed at different positions on the device in which theelectrostatic discharge event occurs are utilized for detecting when theacoustic from the electrostatic discharge event reaches each of theacoustic detectors, and determining the time it takes the acoustics totravel to each of the acoustic detectors with the known positions of theacoustic detectors and utilizing this information to determine thelocation of the electrostatic discharge event within the device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an electrostatic dischargedetector and locator system for carrying out the method; and

FIG. 2 is a schematic circuit diagram for the system used in the methodand illustrating the records caused to be produced by the detectors.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, the event desired to be detected islocated in a device such as structure 10 that contains materials thatare sensitive to electrostatic discharge. Structures of this type arepropellants as well as other types of missile systems. Anelectromagnetic pickup device 12 is secured in a conventional manner tostructure 10 and has an antenna 14 (see FIG. 2) such as a loop or stubantenna for picking up the electromagnetic waves of the event. Antenna14 is connected by leads 16 through a high frequency audio amplifiermeans 18 greater than 100 Khz. Amplifier 18 is connected by appropriatelead 20 to a recording device 22 such as an oscilloscope, oscillograph,or computer to provide a trigger input through lead 20 to recordingdevice 22 to mark the starting of time at t=0 as indicated. Even thoughone electromagnetic pickup device has been illustrated, it may bedesirable in some applications to provide more than one electromagneticpickup on the structure in which the electromagnetic discharge event isto occur. Structure 10 also includes three acoustic detectors 24, 26,and 28 that are attached in a conventional manner to the structure ofthe system under test. These acoustic detectors 24, 26 and 28 arepreferrably contact accelerometers such as high frequency microphonesthat have a frequency response of 2.5 KHz to 35 KHz with a rise time of10⁻⁵ seconds or less. Detectors 24, 26 and 28 are connected by leads 30,32 and 34 to instrumentation control containers 36, 38 and 40. Eachinstrumentation control container includes an accelerometer drive forits respective detector and conditioning amplifier means. The outputfrom the conditioning amplifier means is connected by leads 42, 44, and46 to recording device 22 for recording the acoustics detected bydetectors 24, 26 and 28 to establish times t₁, t₂, and t₃. If desired,instrumentation control containers 36, 38 and 40 can contain fiber opticdrives and fiber optic controls as are conventional in this type art forcoupling the information. It is noted that detectors 24, 26, and 28 havebeen arbitrarily located as indicated, and the indicated locations workwell. Since the location of the electrostatic discharge event relativeto the acoustic detectors is to be determined, it is not desired toplace the acoustic detectors close together or in a straight line. Thefirst signal to reach each of detectors 24, 26 and 28 is the only usefulsignal that is used in determining the location of the electrostaticdischarge event. There may be many standing waves set up in the materialand the second, third, etc., wave may be detected until the signals aredifficult to distinguish from background clutter. These signals may beuseful in establishing the strength of the electrostatic dischargeevent; however, this particular feature is not of particular interest tothis invention.

In operation, when an electrostatic discharge event occurs in structure10, the event is detected at antenna 14 of electromagnetic pickup 12 andthe signal triggers recording device 22 to start the time at t=0 asillustrated. The time required for the signal to travel from thelocation of the electrostatic discharge event to the location of theantenna of the electromagnetic pickup has been neglected. This time isof the order of a few nanoseconds. Some time later, after time t=0, sayt₁, an acoustic signal from the electrostatic discharge event isdetected by the closest detector of detectors 24, 26, and 28. If thevelocity of sound is V, then the distance X from this detector to theelectrostatic discharge event's location is given by X₁ =Vt₁. Therefore,the electrostatic discharge event occurs somewhere on the surface of asphere centered at the location of this particular detector with aradius of X₁. The next detector of detectors 24, 26 and 28 to receivethe signal determines a time t₂ as illustrated and is located distanceX₂ =Vt.sub. 2. This electrostatic discharge event is therefore alsolocated on the surface of a sphere centered at the location of thesecond closest detector with a radius of X₂. Thus, the electrostaticdischarge event is located on a circle which is the innersection ofthese two spheres. The fartherest detector of detectors 24, 26 and 28reoeives the signal at time t₃ and likewise determines a sphere, ofradius X₃ =Vt₃, on the surface of which the eleotrostatic dischargeevent also occurs. The surface of this third sphere also intersects theabove circles at only one point. This common point of the three sphereslocates where the electrostatic discharge event took place.

The location of the electrostatic discharge event is describedmathematically below. The first detector assumes a right handedcoordinate system X, Y, Z, and that t₁ <t₂ <t₃. Let the first detectorbe located at (-h₁, 0, 0), the second detector be located at (h₁, 0, 0),the third detector be located at (0, h₂, h₃). Here the X axis has beenchosen so that both the first detector and the second detector arelocated on it. Now if the event occurs at (X, Y, Z) then the threeshares on which the event is located are given by ##STR1## Where X₁=Vt₁, X₂ =Vt₂, and X₃ =Vt₃ and the solution of this system of equationsgives the values of X, Y, and Z for the electrostatic discharge event.The above equations are quadratic, and therefore there are two possiblesolutions. This ambiguity can be removed by the use of an additionalacoustic detector. However, for most applications the three acousticdetectors can be arranged so that one of these solutions falls outsidethe body being tested. In any event, in practice this data processingcan be accomplished by allowing recording device 22 to be a smallcomputer.

In some systems where several materials are used, there existinhomogenuities and the velocity of sound may not be constant orisotropic. These anisotropic properties can be troublesome, but thesystems of concern are known and a knowledge of the materials involvedexist. Therefore, these uncertainties can be minimized by calibrationtechniques.

We claim:
 1. A method of locating an electrostatic discharge event in apredetermined structure, said method comprising sensingelectromagnetically that an electrostatic discharge event has occurred,producing a trigger signal from said sensed electromagnetic dischargeevent and utilizing said trigger signal to control a recording deviceand start the counting of time at t=0, sensing acoustics from saidelectrostatic discharge event at three spaced apart positions on saidstructure and communicating the sensed acoustics as inputs to saidrecording device to determine the time ellapsed from t=0 until theacoustics from the electrostatic discharge event reaches each of thethree sensing acoustic and utilizing the information gained by theelectromagnetic sensing and the acoustic sensing to determine thelocation of the electrostatic discharge event in said structure.
 2. Amethod of locating an electrostatic discharge event as set forth inclaim 1, wherein an antenna is used in the electromagnetic sensing ofsaid electrostatic charge event, and wherein high frequency microphoneswith frequency response from 2.5 KHz to 35 KHz are used for saidacoustic sensing.
 3. A method of locating an electrostatic dischargeevent as set forth in claim 2, wherein said recording device utilized isan oscilloscope.
 4. A method of locating an electrostatic dischargeevent as set forth in claim 3, wherein said acoustic sensing is sensedat random positions about said structure.